1. Field of the Invention
This invention relates to anodized aluminum substrates for electronic package and printed circuit board applications. More particularly, the breakdown voltage of an anodic film is increased by selection and processing of the aluminum core and by sealing pores in the anodic film.
2. Description of Related Art
Anodized aluminum substrates are used in electronic packaging applications as disclosed in U.S. Pat. No. 4,939,316 to Mahulikar et al. that is incorporated by reference in its entirety herein. Mahulikar et al. disclose an adhesively sealed electronic package having anodized aluminum base and cover components. A leadframe is disposed between and bonded to both the base and the cover. An integrated circuit device is adhesively bonded either to the package base or to an intervening die paddle.
The breakdown voltage is one property of interest to electronic package designers. The breakdown voltage is the voltage that when applied across a dielectric layer causes an irreversible degradation of that dielectric layer and is accompanied by an increase in electrical conductivity of several orders of magnitude.
The breakdown voltage is not critical in electronic packages of the type disclosed in U.S. Pat. No. 4,939,316 because a thick polymer layer separates the circuits and the leadframe from the anodized substrate. However, recent electronic package designs include circuitry deposited directly on the anodization layer by processes as diverse as direct writing, screen printing, electrolytic plating, electroless plating, vaporization, sputtering and vapor deposition. With circuits formed directly on the anodization layer, the breakdown voltage has become much more critical.
Examples of electronic packages having conductive circuitry formed directly on the anodization layer are disclosed in U.S. patent application Ser. No. 08/134,993, entitled "Edge Connectable Electronic Package" by Hoffman et al. that was filed on Oct. 12, 1993 and U.S. patent application Ser. No. 08/277,387 entitled "Integrally Bumped Electronic Package Components" by Mahulikar et al. that was filed on Jul. 19, 1994, both of which are incorporated by reference in their entireties herein.
Maximizing the breakdown voltage is now an objective of electronic package design. Several factors influencing the breakdown voltage are illustrated in FIG. 1. An anodized aluminum component 10 has an aluminum or aluminum alloy core 12 coated with an anodization layer 14 having a typical thickness of from about 0.013 mm to about 0.075 mm (0.0005-0.003 inch). The anodization layer is deposited by an electrolytic process as known in the art. One such process, integral color anodization, is disclosed in U.S. Pat. No. 5,066,368 to Pasqualoni et al. that is incorporated by reference in its entirety herein. An anodization layer is formed by immersing an aluminum alloy in an electrolyte containing 1-4 g/l sulfuric acid and 50-120 g/l sulfosalicylic acid. A direct electric current is then impressed across the electrolyte with the aluminum alloy as the anode for a time effective to form an anodic film of a desired thickness,
Common defects that reduce the breakdown voltage include intermetallics 16 and surface defects 18. Pores 20 trap contaminants 22 which can corrode the anodic film, further reducing the breakdown voltage.
The intermetallics 16 average up to 20 microns in length along the longest axis and may contain non-anodizable, electrically conductive alloy constituents such as Al--Fe--Si, FeAl.sub.3 and MnAl.sub.6.
The anodic film 14 is thin and surface defects are not completely filled in. The result is a crevice or pit that reduces the breakdown voltage.
Increasing the thickness of the anodic film does increase the breakdown voltage, but is not an effective solution. A thick anodic film reduces the dissipation of heat through the film to the aluminum core and increases the propensity of the anodic film to cracking. Additionally, increasing the film thickness increases the anodization time and power requirements, increasing the cost of the component.
To maximize the breakdown voltage, it is therefore desired to reduce the above stated defects as well as others.